What is the relationship between this word and Elf-king, I wonder?

2016.02.04 I answer myself: both words refer to the King of the Mirkwood Elves (Thranduil, except possibly the very first reference  in chapter 8).  “Elvenking” is used exclusively in Chapter 9 and onward; “Elf-king” is used only in Chapter 8.

• 09.005 sat the Elvenking on a chair of carven wood.
• 09.011 about the Elvenking’s palace.
• 10.033 by the Elvenking
• 10.034 The Elvenking was very powerful
• 14.040 The Elvenking had received news
• 14.040 Even before the Elvenking rode forth
• 14.042 in return for the Elvenking’s aid.
• 14.044 and the most of the Elvenking’s array,
• 15.027 that the Elvenking had turned aside to the Lake,
• 15.044 They bore with them the green banner of the Elvenking
• 15.053 Nor at all with the people of the Elvenking,
• 15.054 The Elvenking is my friend,
• 16.025 both the Elvenking
• 16.036 The Elvenking himself,
• 16.040 The Elvenking looked at Bilbo with a new wonder.
• 17.003 and the Elvenking,
• 17.037 But the Elvenking said:
• 17.045 in council with the Elvenking
• 17.060 he preferred on the whole to defend the Elvenking.
• 18.032 Upon his tomb the Elvenking then laid Orcrist,
• 18.034 To the Elvenking he gave the emeralds of Girion,
• 18.043 Gandalf and Bilbo rode behind the Elvenking,
• 18.044 Farewell! O Elvenking!’